1. Your mysql server is down. Can you access it via phpmyadmin through your hosting control panel?
2. If the database is up, then you have an incorrect value in one of the DB_ defines in wp-config.php. Verify the database name, user, password, and host. If you don’t know what these values should be, contact your host.
If the site used to work and now doesn’t, either someone changed something (in wp-config.php or at the database level) or the mysql server is down.
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